Identify and Manage
Hazardous Defects in
Your Trees
4
Prevent personal injury and property damage
In Your Backyard Woods
Trees are key to the enjoyment of your backyard woods.
Structurally defective trees; however, can fail and cause
personal injury and property damage. A tree with structural defects that are likely to cause failure is considered a
“high risk or hazardous tree” if it could strike a target. A
target can be a vehicle, building, or a place where people
gather such as a bench, picnic table, trail, or fire pit.
tree, including the roots, root or trunk flare, main stem,
branches, and branch unions. Be sure to examine all sides
of the tree. Use binoculars to see high branches. Consider
the following factors:
Many structural defects that contribute to tree failure can
be prevented through proper tree planting and pruning
practices. (See Backyard Woods Tip Sheets on Plant Trees
and on Prune Your Trees for more information.)
Tree condition
Trees in poor condition may have many dead twigs, dead
branches, or small, off-color leaves. Trees in good condition will have full crowns, vigorous branches, and healthy,
full-sized leaves; however, green foliage in the crown does
not ensure that a tree is safe. Tree trunks and branches can
be defective and still support a lush green crown.
To ensure your family and friends have a safe environment
in which to enjoy the beauty and many benefits of your
backyard woods, you can learn to recognize hazardous
defects in trees and take corrective actions. At the same
time, you can increase wildlife habitat, aesthetic value, and
recreational opportunities within your backyard woods.
Tree species
Certain tree species are prone to specific types of defects.
For example, some species of maple and ash in the
Northeast often form weak branch unions, and aspen is
prone to breakage at a young age (50-70 years) due to a
variety of factors, including decay and cankers.
A professional forester or arborist should undertake many
of the suggested corrective actions. Because of the natural
variability of trees, the severity of their defects, and the
different sites on which they grow, evaluating trees for
hazardous defects can be a complex process. The following are guidelines, not absolute rules, for recognizing and
correcting high-risk tree defects. When in doubt, consult a
professional forester or arborist.
Tree age and size
Trees are living organisms subject to constant stress. Pay
particular attention to older trees, which may have accumulated multiple defects and extensive decay.
Inspecting Trees
Trees that are in high use areas and within striking distance of a target, should be inspected every year and after
severe storms. This usually includes all trees within your
immediate backyard, and along trails, near picnic areas
or fire pits, or campsites within your backyard woods.
These inspections will allow you to detect defects and
correct them before they pose significant risks to personal
safety and property. Tree inspections can be done at any
time of year, with or without leaves present. Inspect trees
carefully and systematically. Examine all parts of the
Defects to Look For
High-risk defects are visible signs that a tree is failing.
Look for these seven main types of tree defects: dead
wood, cracks, weak branch unions, decay, cankers, root
problems, and poor tree form. These seven main types of
tree defects are described below. Remember: a tree with
defects is not hazardous unless some portion of it is within
striking distance of a target.
Dead wood
Dead trees and branches are unpredictable and can break
and fall at any time. Dead wood is often dry and brittle
and cannot bend in the wind like a living tree or branch.
A dead branch and treetop that is already broken off
(“hanger” or “widow maker”) is especially dangerous.
Act immediately if . . .
• A broken branch or treetop is lodged in a tree.
• A tree is dead.
• A branch is dead and of sufficient size to cause injury.
Typically this is a branch larger than 4 inches in
diameter, but this can vary with branch height. Even
smaller branches, falling from high in the tree, can
cause serious personal injury.)
Dead branches can break and fall at any time.
Cracks
A crack is a deep split through the bark, extending into the
wood of the tree. Cracks are extremely dangerous because
they indicate that the tree is already failing.
Act immediately if . . .
• A crack extends deeply
into, or completely through
the stem.
• Two or more cracks
occur in the same general
area of the stem.
• A crack is in contact with
another defect.
• A branch of sufficient
size to cause injury is
cracked (typically larger
than 4 inches in diameter).
This weak branch union has
developed a crack, creating a
highly hazardous situation.
2
Weak Branch Unions
Weak branch unions are
places where branches
are not strongly attached
to the tree. A weak union
occurs when two or more
similarly sized, usually
upright branches grow
so closely together that
bark grows between
the branches, inside the
union. This ingrown bark
(included bark) does not
have the structural strength
of wood, and the union is
much weaker than one that
does not have included
bark. The included bark
may also act as a wedge
and force the branch union
A weak branch union with
to split apart. Trees with a
included bark and a crack.
tendency to form upright
branches, such as elm and maple, often produce weak
branch unions. Weak branch unions also form after a tree
or branch is tipped or topped, that is, when the main stem
or a large branch is cut at a right angle to the direction of
growth leaving a large branch stub. The stub inevitably
decays, providing very poor support for new branches
(“epicormic” branches) that usually develop along the cut
branch.
Act immediately if . . .
• A weak branch union occurs on the main stem.
• A weak branch union is associated with a crack, cavity,
or other defect.
Decay
Decaying trees can be prone to failure, but the presence of
decay alone does not indicate that the tree is hazardous.
Advanced decay (wood that is soft, crumbly, or a cavity
where the wood is missing) can create a serious hazard.
Evidence of fungal activity including mushrooms, conks,
and brackets growing on root flares, stems, or branches are
indicators of advanced decay.
A tree usually decays from the inside out, eventually forming a cavity, but sound wood is also added to the outside
of the tree as it grows. Trees with sound outer wood shells
may be relatively safe, but this depends upon the ratio of
sound wood to decayed wood, and other defects that might
be present. Evaluating the safety of a decaying tree is
usually best left to trained arborists.
Act immediately if . . .
• Advanced decay is associated with cracks, weak
branch unions, or other defects.
www.arborday.org/backyardwoods
This tree should have been evaluated and removed before it
failed. Detectable signs of wood decay were present including a
main stem cavity with soft, crumbly wood.
• A branch of sufficient size to cause injury is decayed.
• The thickness of sound wood is less than 1inch for
every 6 inches of diameter, at any point on the stem.
Cankers
A canker is a localized area on the stem or branch of a
tree, where the bark is sunken or missing. Cankers are
caused by wounding or disease. The presence of a canker
increases the chance of the stem or branch breaking near
the canker. A tree with a canker that encompasses more
than half of the tree’s circumference may be hazardous
even if exposed wood
appears sound.
This tree has a severe lean and much of the root system has
been recently exposed.
often associated with root problems. Because most defective roots are underground and out of sight, aboveground
symptoms may serve as the best warning.
Act immediately if. . .
• A tree is leaning with recent root exposure, soil movement, or soil mounding near the base of the tree.
• More than half of the roots under the tree’s crown
have been cut or crushed. These trees are dangerous
because they do not have adequate structural support
from the root system.
• Advanced decay is present in the root flares or “buttress” roots.
Act immediately if...
• A canker or multiple
cankers affect more than
half of the tree’s circumference.
The canker and associated
wood decay have seriously
weakened the stem of this
birch tree.
• A canker is physically
connected to a crack,
weak branch union, cavity, or other defect.
Root Problems
Trees with root problems may blow over in windstorms.
They may even fall without warning in summer from the
weight of the tree’s leaves. Many actions can cause root
problems such as severing or paving-over roots; raising
or lowering the soil depth near the tree; parking or driving vehicles over the roots; or extensive root decay. Soil
mounding, twig dieback, dead wood in the crown, and
off-color or smaller than normal leaves are symptoms
Poor Tree Form
Poor tree form is a growth pattern that indicates weakness
or structural imbalance. Trees with strange shapes are
interesting to look at, but may be structurally defective.
Poor tree form often results from many years of damage
from storms, unusual growing conditions, improper pruning, topping, and other damage.
A leaning tree may be a hazard. Because not all leaning
trees are dangerous, a professional arborist should examine
any leaning tree of concern.
Act immediately if . . .
• A tree leans excessively.
• A large branch is out of proportion with the rest of the
crown.
3
Prune the Tree
A defective branch or branches may cause a high-risk
situation, even though the rest of the tree is sound. In this
case, pruning the branch or branches solves the problem.
Practicing proper tree pruning is excellent “preventive
medicine” for reducing the occurrence of defective, highrisk trees. By pruning properly, early in a tree’s life, you
can prevent or correct many of the problems that cause
trees to fail as they mature.
Improper pruning techniques can worsen the problem, and
may ultimately result in the formation of cracks, decay,
cankers, or poor tree architecture that lead to very high risk
situations. (See the Backyard Woods Tip Sheet on Prune
Your Trees for more information.)
Convert the Tree to a Wildlife Tree
Creating wildlife habitat is a corrective action that is often
overlooked. Corrective actions should be selected with
safety first and foremost; however, options exist that can
This tree has multiple defects present
(advanced wood decay with fungal conks,
and a large crack) and should be removed.
Multiple Defects
The recognition of multiple defects in a tree is critical
when evaluating its potential to fail. Multiple defects that
are touching or are close to one another should be carefully
examined. If more than one serious defect occurs on the
tree’s main stem, you should assume that the tree poses a
very high level of risk.
Corrective Actions
Corrective actions begin with a thorough evaluation. If
a high-risk situation exists, there are four recommended
options for correcting the problem: move the target, prune
the tree, convert the tree to a wildlife tree, or remove the
tree.
Move the Target
Moving the target is often an inexpensive, effective, and
simple treatment for correcting a high-risk tree. Easily
moved items like play sets and swings, vehicles, and picnic
tables can be placed out of the reach of the hazardous tree
with little effort and expense.
If the target cannot be moved and a high-risk situation
exists, consider blocking access to the target area until the
hazard can be eliminated.
4
Leave this wildlife tree if it will not endanger a target, the target
can be moved, or the tree can be reduced in size to prevent
damage to a target.
www.arborday.org/backyardwoods
successfully reduce safety risks and also leave at least a
portion of the tree intact to provide wildlife habitat. One
option is to reduce the height of a tree to the point that it
would no longer strike a target if it should fail. Removing
major branches that are structurally defective, and leaving
a portion of the tree intact for wildlife habitat may also be
possible. For valuable wildlife trees, consider redirecting
a trail or walkway so that it is located outside the striking
distance of the tree. If a tree must be removed for safety
reasons, consider leaving the tree on ground to create
wildlife habitat.
Remove the Tree
Before cutting a tree down, carefully consider the alternatives. The effects of removing a tree are often pronounced
in landscape situations and may result in reduced property
values. Tree removal should be considered as the final
option and used only when the other corrective actions
will not work. Tree removal is inherently dangerous and
is even more serious when homes and other targets are
involved. Removal of very high-risk trees is usually a job
for a professional arborist.
Cabling and Bracing
Cabling and bracing do not eliminate all the safety risks
associated with a highly defective tree, but when done correctly by a trained arborist, it can extend the time a tree or
its parts are safe. Done incorrectly, it creates a more serious hazard. Cabling or bracing is not recommended for
a high-risk tree unless the tree has significant historic or
landscape value, the cabling or bracing is done by a trained
arborist, is regularly inspected, and properly maintained.
Remember that trees do not live forever. Design and
follow a backyard landscape plan that includes proper tree
selection, and a cycle of tree maintenance and replacement. (See Backyard Woods Tip Sheet on Make a Master
Plan for more information.) For backyard woodlot management, see the Backyard Woods Tip Sheet on Help Your
Preferred Trees Grow for information on how to select
trees with the greatest potential to accomplish your goals,
whether they include improving wildlife habitat, enhancing
the beauty of the woods, increasing recreational opportunities, or improving timber quality.
In the Forest
The USDA Forest Service and the U.S. Department
of Interior’s National Park Service manage developed
recreational areas within the National Forests and Parks
to help ensure public safety. Both Federal agencies
have established hazard tree management policies and
standards, requiring periodic, thorough, and documented
tree inspections of developed recreation sites. Many
State Forests and parks have similar tree risk management
plans. Just as the management practices of National and
State Forests and Parks help to ensure public safety, your
backyard woods should be properly managed to ensure a
safe environment for your family and friends.
Conclusions
Evaluating and treating high-risk trees can be a complicated process, requiring a certain level of knowledge and
expertise. This publication outlines some of the basic
problems that may alert you to a hazardous situation, and
suggests some possible ways to correct them. A professional forester or arborist should undertake many of the
suggested corrective actions. When in doubt about how
much risk a defective tree poses, or how to best treat it,
consult a professional forester or arborist. You may wish
to contact the local Cooperative Extension Office for educational assistance and to inquire if a listing of qualified
consulting foresters or arborists is available for your area.
Otherwise, consult your phone book under “Arborists” or
“Tree Service.”
5
Suggested Reading
Acknowledgments
Albers, J.; Hayes, E. 1993. How to detect, assess and
correct hazard trees in recreational areas, revised edition.
St. Paul, MN: Minnesota DNR; 63 p.
Information in this tip sheet is a modified version of the
publication entitled How to recognize hazardous defects in
trees. NA-FR-01-96. Radnor, PA: USDA Forest Service,
Northeastern Area State and Private Forestry; 19 pp. The
publication is also available at: http://www.na.fs.fed.us.
Bedker, P.J.; O’Brien, J.G.; Mielke, M.E. 1995. How to
prune trees. NA-FR-01-95. Radnor, PA: USDA Forest
Service, Northeastern Area State and Private Forestry; 30
p. Also available at: http://www.na.fs.fed.us.
Fazio, J. 1989. How to hire an arborist. Tree City USA
Bulletin No. 6. Nebraska City, NE: National Arbor Day
Foundation; 8 p.
Fazio, J. 1989. How to recognize and prevent hazard
trees. Tree City USA Bulletin No. 15. Nebraska City,
NE: National Arbor Day Foundation; 8 p.
Robbins, K. 1986. How to recognize and reduce tree
hazards in recreation sites. NA-FR-31. Radnor, PA:
USDA Forest Service, Northeastern Area; 28 p.
Shigo, A. L. 1986. A new tree biology. Durham, NH:
Shigo and Trees, Associates; 595 p.
A Cooperative Project of:
USDA Forest Service
NORTHEASTERN AREA
State and Private Forestry
www.na.fs.fed.us
6
National Association of
Conservation Districts
www.arborday.org/backyardwoods